Enhanced quantum radiation with flying-focus laser pulses

TL;DR

The paper addresses enhancing quantum radiation reaction signals in strong-field QED by comparing flying-focus (FF) pulses to stationary-focus Gaussian (SFG) pulses at equal energy. It develops a LCFA-based description of energy loss and photon emission, introducing the relative loss $\zeta$ and analyzing photon-yield scaling with interaction time, highlighting the time-driven advantages of FF. The authors show that FF pulses yield greater electron energy loss and higher photon yield, particularly in the 1–20 MeV range, due to extended interaction times, with the quantum regime ($\chi \gtrsim 1$) exhibiting strong time scaling. Validation via Monte Carlo SFQED and PIC simulations confirms the FF advantage and supports practical benefits for gamma-ray sources and SFQED experiments, including lower power requirements and simpler diagnostics of field strength.

Abstract

The emission of a photon by an electron in an intense laser field is one of the most fundamental processes in electrodynamics and underlies the many applications that utilize high-energy photon beams. This process is typically studied for electrons colliding head-on with a stationary-focus laser pulse. Here, we show that the energy lost by electrons in the quantum regime and the yield of emitted photons can be substantially increased by replacing a stationary-focus pulse with an equal-energy flying-focus pulse whose focus co-propagates with the electrons. These advantages of the flying focus result from the energy loss and the photon yield scaling more favorably with the interaction time than the laser intensity in the quantum regime, with the latter also holding in the classical regime. Monte Carlo simulations of electrons colliding with equal-energy stationary and flying-focus laser pulses demonstrate these advantages.

Figures (4)

• Figure 1: Relative energy loss $\zeta$ as a function of the interaction time for an electron colliding head-on and on-axis with equal-energy FF and SFG pulses. From top to bottom, the lines correspond to initial electron energies of 10, 5, 2, 1, and 0.5 GeV . Both the FF and SFG pulses had a $\lambda_0 = 1\ \mu$m wavelength, $\sigma_0 = 1.5\ \mu$m focal spot size, maximum quantum nonlinearity parameter $\chi_0 \approx 0.1 \xi_0 \mathcal{E}_0$(10 GeV), and energies of either (a) $U=1\;\text{J}$ or (b) $U=10\;\text{J}$. For the SFG, $z_R = 42$ fs. To the right of the dash-dotted vertical line, the results may have errors larger than 4% due to the breakdown of the LCFA.
• Figure 2: Relative energy loss $\zeta$ as a function of the initial electron energy for electrons colliding head-on and on-axis with equal-energy FF and SFG pulses. In (a), the pulses have an energy $U=$ 1 J, 10 J, or 100 J and a spot size $\sigma_0=$1.5 $\mu$m. In (b), $U=$ 10 J and the spot size is $\sigma_0=$ 1.5 $\mu$m or 4 $\mu$m. The color scale shows the maximum quantum nonlinearity parameter. Above $\mathcal{E}\approx$ 3 GeV, both the FF and SFG are in the quantum regime. The inset shows the relative improvement in the energy loss afforded by the FF pulses. At $U=$ 1 J, the FF results in ${\sim}30\%$ more loss than the SFG. At $U=$ 10 J, both the FF and SFG begin to approach $100\%$ loss, which limits the relative improvement achievable with a FF pulse.
• Figure 3: Number of photons emitted per electron at $\chi > 0.1$ from the on-axis collision of $\mathcal{E}_0 = 10$ GeV electrons with 1 J FF and SFG laser pulses. The inset shows the number of photons emitted as a function of energy and $\eta_\text{LCFA}^{2/3}$ for the FF pulse. The numbers in white indicate what percentage of photons in the given range were emitted at $\chi > 0.1$.
• Figure 4: Spectra of emitted photons from the head-on collision of $\mathcal{E}_0 = 10$ GeV and 1 GeV electrons with $U = 10$ J laser pulses. The inset shows the number of photons emitted as a function of energy and $\eta_\text{LCFA}^{2/3}$ for the FF pulse collisions. The number of photons emitted per electron in each shaded interval is displayed in the bottom left. The FF pulse (solid lines) results in approximately five-times and three-times more photons in the 1--20 MeV range than the SFG pulse (dashed lines) for 10 and 1 GeV electrons, respectively.